I'm assuming you're running on a TUN interface, and not TAP. Therefore,before you try David's suggestions, a couple of thoughts:

1) do the systems on the SERVER side of the network have a route back tothe CLIENT network (either by having a route in their own routing tableto route the client network via the "server"'s IP. This can even bedone at the firewall, assuming it'll send out ICMP redirects).

2) Assuming the server is a Linux/Unix box... Do you have IP forwardingenabled? Under Linux, check that /proc/sys/net/ipv4/ip_forwarding isset to 1.